Field of the Invention
Embodiments of the present invention relate to a projection system, a lithographic apparatus, a method of projecting a beam of radiation onto a target and a method for manufacturing a device.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. By contrast, angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
Such optical scatterometers may be used to measure parameters, such as critical dimensions of developed photosensitive resist or overlay error (OV) between two layers formed in or on the patterned substrate. Properties of the substrate can be determined by comparing the properties of an illumination beam before and after the beam has been reflected or scattered by the substrate. Asymmetries in the shape of a target grating will generally have an impact on the measured overlay. This impact may vary depending on the illumination setting used for the measurement.
A previous technique of OV measurement evaluated difference in intensities of diffraction orders. Such a technique is detailed in US 2012/0013881 A1, which is incorporated by reference in its entirety. A similar technique, disclosed in US 2001/0027704 A1, which is incorporated by reference in its entirety, uses parallel illumination at multiple wavelengths. In both cases, symmetry of the combined illumination beams is important for OV measurement accuracy. The asymmetry of the diffraction orders is a measure of overlay on the target. One problem of these techniques is high sensitivity to non-uniformity and asymmetry of the illumination beam in the pupil plane. Some systems use set of changeable apertures in the pupil plane, but requirements for positioning accuracy of these apertures are extremely tight. New demands for OV accuracy have increased for new generation of lithography metrology tools make this problem extremely important.